Brake Issue - 2007 Tahoe - 106,000 Miles

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Brake pedal went almost to the floor before brakes engaged.
Until I was able to get off road brake pedal was mushy and I had to pump the brakes up to get them to engage.
After getting car off the road I expected to find a fluid leak and low reservoir but found no leak and reservoir was full.
Started engine and brakes worked great, drove home without another incident, brakes worked great.
Checked brakes again this morning, they worked great, no leaks, plenty of fluid, no problem.
What caused the original brake failure that went away on its own.
Thanks in advance for the help.

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I was positive it was the master cylinder and I could have changed it myself but I wanted to have it checked out by someone who hopefully knows more about it than I do, the last thing I want to happen is lose the brakes especially when I'm towing.
Took it to the Chevy dealer and they confirmed the master cylinder.
Thanks for all the input.

[John_Wayne]

Take it to a shop that just does brakes and let them check it for you it some cases the check may even be free.

[time2roll]

Yes MC can have an intermittent issue. DW had this on a Nissan. Waited for the second scare to have it replaced. No telling what the third could have been. Had no issues once replaced.

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eHoefler wrote:
MASTER CYLINDER IS BYPASSING, REPLACE IMMEDIATLY, DO NOT DRIVE UNTIL REPLACED!

I tend to agree but what is confusing me is at the moment it is working just fine.

If the master cylinder was bypassing wouldn't that mean that there is damage internally and from what I understand a defective master cylinder normally does not malfunction intermittently, when its broke, its broke.

[eHoefler]

MASTER CYLINDER IS BYPASSING, REPLACE IMMEDIATLY, DO NOT DRIVE UNTIL REPLACED!

[Macrosill]

I had the same symptoms but in a motorhome. Scariest drive of my life. Replaced the front calipers right away. Never happened again. This usually happens on vehicles that sit for extended periods of time. Especially if there is a tear or rip in the caliper piston boot. The caliper rusts as it sits. Then it extends under braking and can not retract properly causing it to overheat.

Sounds like the perfect time for a brake inspection, overhaul and flush.

[time2roll]

Replace the master cylinder with new. Don't wait for it to become a consistent problem. Next issue could be at a critical moment.
All brake fluid should be bled/flushed at time of service.

[Campfire_Time]

fitznj wrote:


Try bleeding the brakes and see if you spot any air bubbles.

That would be my suggestion.

[fitznj]

Brake pedal should go down slightly when the engine is turned on. From
what you've written, it sounds like the master cylinder is okay;

My next thought is that maybe you overheated the brakes prior to your incident. This would have raised the temp of the brake fluid.

Try bleeding the brakes and see if you spot any air bubbles.

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Disc brakes all around, pads are not warn and rotors are in excellent condition.

With engine off brake pedal locks up when pushed and does not sink when held down.

With engine running brake pedal goes further down when pushed but still locks up and does not sink when held down.

Break fluid appears clean and normal color, with no leaks and a full reservoir.

I don't know if it is normal for the break pedal to compress further down when the engine is running but I believe it always did.

Keep in mind that since the incident yesterday the breaks worked fine all the way home, about 30 miles in stop and go traffic and still work fine this morning.

I want to figure this out now and not when Im towing.

[ChuckSteed]

I’d have to be taking a look at power brake vacuum booster and vacuum lines for issues...at 107K if the brakes have not been replaced on a 2007 Tahoe they have to be worn way down along with the rotors...

[fitznj]

I've had this happen more than once on older cars - classic sign
that you master cylinder needs to be replaced or rebuilt.

If your brake fluid is turning black, then the rubber cups are breaking down. This will let fluid past them when you press the
brake pedal but you are not leaking brake fluid. (do a google search)

Do the following:
with the Ignition off put very slight pressure on the brake pedal,
does the brake pedal slowly sink? if so then your master cylinder
needs replacing. (this test may take a minute or 2)

Try the same test with the engine running.

If you have brake drums - remove them and see of there is dampness
around the wheel cylinders.

[wolfe10]

Guess I will start the "speculation":

If one brake caliper did not properly release, it could severely overheat. Particularly if brake fluid is not new, this can cause the brake fluid to BOIL.

That would cause your symptoms.

So, check all for brakes carefully for sign of overheating and also change your brake fluid.